There is a growing suspicion that the viruses to which laboratory workers are exposed by their occupation may cause cancer in humans. No proof of this belief is available at present. Before undue alarm is created among the exposed populations and before expensive and elaborate protective devices are required in laboratories, it would be helpful to investigate this problem with careful epidemiological studies. Our preliminary evidence from proportional mortality data and from case-control studies of physicians in several professional societies has yielded conflicting results. There is some evidence that the physicians specializing in research with certain viruses may have an associated risk of cancer death but the numbers are too small to reach a definitive conclusion. The proposed research would expand the population base and investigate through two non-concurrent prospective studies the risk of cancer deaths in both self-classified virologists and in societies which have a large enrollment of virologists and compare the results for these groups with appropriate controls. The exposure to known oncogenic animal viruses in each of these groups would be carefully examined. If a definite risk is noted in occupationally exposed groups, the finding could lend strong support to the popular theory that some cancers are produced by viruses.